Functional independence of tree branches under long-term ozone stress: Gas exchange and chlorophyll fluorescence

The use of branches in air pollution research was evaluated with a test of the hypothesis that branches can be treated as functionally independent units. Field-planted Douglas-fir saplings were enclosed in open-top chambers with branches at an upper and lower position in the crown enclosed in open-end branch chambers. The pollutant environment of branches could be manipulated independently of that of the tree. Comparisons were made between branches with the same treatment but different whole tree treatments. For current foliage exposed to ozone net photosynthesis and photochemical efficiency were reduced while CO{sub 2} compensation point and stomatal and non-stomatal inhibition of net photosynthesis were increased. First year results support the hypothesis of functional independence. Second year results suggest a limit to functional independence with branches largely, but not completely, independent.